The subject field is mostly useless. Few people use it, and when it's used it's often annoying. The password field is just confusing and nobody really uses it anyway. In Kareha, it's hidden and auto-generated, but I guess this could be further refined, for instance so that a post with an empty password (in case Javascript is off) can be deleted from the same IP only. I am a bit torn on the "No file" checkbox. On the one hand, it serves a purpose by warning people when they try to reply without pressing Reply, but on the other hand people still manage to do that.
If it wasn't for sage, the link field would also be mostly useless. If I was starting from scratch, replacing it with a checkbox for sage might be a better choice. Not to worry, though, I know lots of people don't like that, and I'm not about to do it.